This invention relates to a novel electrodeionization apparatus and method adapted to transfer ions in a liquid under the influence of a polar field. More specifically, this invention relates to an electrodeionization apparatus and method adapted to purify aqueous liquids to effect the production of high purity water.
The purification of a liquid by reducing the concentration of ions or molecules in the liquid has been an area of substantial technological interest. Many techniques have been used to purify and isolate liquids or to obtain concentrated pools of specific ions or molecules from a liquid mixture. The most well known processes include electrodialysis, liquid chromatography, membrane filtration and ion exchange. A lesser known methodology is electrodeionization, occasionally mistermed filled cell electrodialysis. Although electrodeionization has the potential to be quite effective in removing ions from liquid, it has never been developed to the degree that it is competitive either structurally or operationally with the better known separation techniques. This is due primarily to the inconsistencies of structural design and unpredictable variances incurred by the presently known modes of use. This lack of structural design precision and nonpredictability of results have reduced the use of electrodeionization to the point where it is relatively unknown even to practitioners skilled in separation methodologies.
The first apparatus and method for treating liquids by electrodeionization was described by Kollsman in U.S. Pat. Nos. 2,689,826 and 2,815,320. The first of these patents describes an apparatus and process for the removal of ions within a liquid mixture in a depleting chamber through a series of anionic and cationic diaphragms into a second volume of liquid in a concentration chamber under the influence of an electrical potential which causes the preselected ions to travel in a predetermined direction. The volume of the liquid being treated is depleted of ions while the volume of the second liquid becomes enriched with the transferred ions and carries them in concentrated form. The second of these patents describes the use of macroporous beads formed of ion exchange resins as a filler material positioned between the anionic or cationic diaphragms. This ion exchange resin acts as a path for ion transfer and also serves as an increased conductivity bridge between the membranes for the movement of ions. These patents represent the primary structural framework and theory of electrodeionization as a technique. The term electrodeionization refers to the process wherein an ion exchange material is positioned between the anionic and cationic diaphragms. The term electrodialysis relates to such a process which does not utilize ion exchange resins between the anionic and cationic diaphragms. Despite the fact that the Kollsman techniques has been available for over 25 years, this technology has not been developed even to the point of practical use. This is due in no small part to the lack of structural designs and the unavailability of operational mode parameters which afford reliable operation of the electrodeionization apparatus. Illustrative of prior art attempts to use the combination of electrodialysis and ion exchange materials to resins to purify saline from brackish water are described in U.S. Pat. Nos. 2,794,777; 2,796,395; 2,947,688; 3,384,568 and 4,165,273. Attempts to improve electrodeionization apparatus are shown in U.S. Pat. Nos. 3,149,061; 3,291,713; 3,515,664; 3,562,139; 3,993,517 and 4,284,492.
Despite the contributions represented by these patents, this prior art has not produced reliable electrodeionization apparatus. This typical resin fouling and membrane scaling problems of electrodeionization remain unalleviated. These electrodeionization apparatus remain unsuitable for desalination or for the production of high purity water. Hard waters, silica-containing waters and highly saline brackish waters, and waters containing colloidal particles and foulants still represent liquids that cannot be consistently and reliably purified by presently known electrodeionization apparatus and modes of operation. Extensive maintenance and cleaning of these apparatus remain necessary, the quality and volume of the purified liquids remain erratic and the ability to product at least 1 meg-ohm centimeter quality water consistently and in sufficient volume remain unachieved.
U.S. Pat. No. 4,632,745 describes an electrodeionization apparatus depletion compartments divided into subcompartments having a width between 0.3 and 4 inches and a thickness between 0.05 and 0.25 inch. It was found that when utilizing this apparatus, efficient ion removal from the depletion compartment is attained while requiring only low energy and while avoiding channeling.
The electrodeionization apparatus of U.S. Pat. No. 4,632,745 can employ one or more, e.g. two hydraulic stages with two separate independent electrical stages. An hydraulic stage comprises a given number, e.g. 30, of cell pairs in which a volume of water flows through. A cell pair comprises a cation and anion membrane which are bonded to a dilute spacer containing ion exchange resin as well as a concentrate spacer. An electrical stage comprises one anode and one cathode electrode which enclose the hydraulic stage. A one-stage stack design is employed for low salinity feed water, whereas a two-stage stack design can be employed for high salinity water. The purpose of a two-stage design is to obtain the maximum salt removal in the first stage without inducing polarization which is an inefficiency resulting in stream pH shifts. The upward pH shift of the concentrate stream will result in scaling if calcium or magnesium ions are present in the feed water to the stack. Maximum salt removal in the first stage without inducing polarization is about 80 percent. The product water from the first stage is fed into the second stage where the remaining salt (about 20%) is removed to obtain meg-ohm product quality. The water bring deionized is passed between each cell pair only once without changing flow direction. The overall design guards against scaling in the first stage while optimizing the overall salt removal, membrane utilization and energy requirement. In comparison, electrodialysis cannot product meg-ohm quality water because it is inefficient in the low salinity range and requires more membrane area and energy. It would be desirable to provide an electrodeionization apparatus and method which provides improved ion removal efficiency as compared to presently available apparatus.